Gypsum Structure
Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay
Gypsum consists of two fundamental structural units: the ubiquitous SO4 tetrahedron and a calcium dodecahedron. The Ca dodecahedron has the formula CaO6.2H2O. The Ca dodecahedron is a common irregular coordination polyhedron that can be pictured as a pentagonal dipyramid split open along two edges with an extra edge inserted to fill the gap.
The view above shows the sulfate tetrahedra in yellow and the Ca dodecahedra in blue (foreground) and green (background)
The view above shows oxygen atoms in blue and hydrogens in red. Oxygen atoms in water molecules are lighter. Calcium atoms are shown within the Ca dodecahedra in purple. The opposite ends of the Ca dodecahedra are shared edge to edge with sulfate tetrahedra. The two remaining oxygens in each Ca dodecahedron are shared with an apex of a sulfate tetrahedron. Thus, all the oxygens are shared with a sulfate tetrahedron, or, if you prefer, the calcium atoms are confined by neighboring sulfate tetrahedra. The two remaining vertices of the Ca dodecahedron are occupied by water molecules.
Calcium polyhedra link edge to edge to form zigzag chains which are cross linked by sulfate tetrahedra. The Ca chains run along the c-axis. This view looks down the b-axis (the two-fold symmetry axis).
This view looks along direction (101), that is, parallel to the b-axis and midway between the a and c axes. The calcium and sulfate layers are separated by hydrogen atoms, in red. We might surmise, correctly, that gypsum has excellent cleavage between the layers.
The view above shows H-O bonds in dark purple, and hydrogen bonds to opposing sulfate oxygens in lighter purple. The hydrogen bonding helps explain gypsum's softness. Although, just ask anyone on the Titanic about the strength of hydrogen bonds!
The view above, down a rather obscure crystallographic direction (411), looks along the three-fold symmetry axes of the sulfate tetrahedra. That, of course, has nothing to do with the gross symmetry of gypsum, which is monoclinic, but it does provide a nice overview of what holds gypsum together. We see the Ca dodecahedra joined by sulfate tetrahedra, with hydrogen atoms extending into the inter-layer spaces. Only the H-O bonds are shown, in purple. The leftmost layer omits some sulfate tetrahedra to better show the linkage of the calcium polyhedra.
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Created 22 Sept 1997, Last Update